Nook stores your folders, pinned tabs, theme choice, and — for the cross-device Today list — the titles and URLs of your open tabs in Firefox's built-in extension sync storage (storage.sync). Firefox syncs that data through your Firefox account like any other synced data. Nook has no servers of its own; nothing is ever sent to the developer or any third party. The optional "auto colour" theme takes a screenshot of the active tab locally to compute an average colour for the sidebar tint; the image is processed in memory on your device and discarded immediately. Local backups never leave your device.